1. Field of the Invention
The present invention relates to an optical apparatus, such as a camera, a lens unit, or an interchangeable-lens camera system, having a correction function to increase a gain (hereinafter referred to as an “ISO (International Organization for Standardization) speed rating”) of a camera to compensate for problems such as a dark object and insufficient light available.
The present invention further relates to a camera apparatus which prevents an increase in noise caused by an increase in a gain of the optical apparatus.
2. Description of the Related Art
In general, an amplification gain is increased for an input signal which is small in amplitude to process an analog signal. A simple amplification also causes amplification of noise components. An amplified noise component is superimposed on a true signal component, and it is therefore difficult to extract a signal component having a high signal-to-noise (S/N) ratio.
The same problem arises for image information obtained by recent digital cameras. Japanese Patent Laid-Open No. 2004-247872 discloses an apparatus adapted to reduce noise in image data. In the disclosed apparatus, since setting the ISO speed rating high causes an increase in noise components in a photographed image, a correction value for reducing noise is changed according to the set ISO speed rating.
To solve problems other than noise for image data, Japanese Patent Laid-Open No. 2005-292740 discloses a device for automatically changing an ISO speed rating in a camera having a plurality of shooting modes.
Japanese Patent Laid-Open No. 2005-292740 focuses on prevention of camera shake. In a shooting mode in which camera shake may occur, the ISO speed rating is set high to prevent camera shake.
In the technique disclosed in Japanese Patent Laid-Open No. 2004-247872, however, if an image is captured in an environment with a lot of noise, in the image, signal components are not sufficiently distinguished from noise components. Thus, the noise components may be superimposed on the photographed image.
In the technique disclosed in Japanese Patent Laid-Open No. 2005-292740, due to the high priority placed on camera shake prevention, the ISO speed rating may be set too high and noise may be superimposed over the entirety of a photographed image.
To avoid this situation, as disclosed in Japanese Patent Laid-Open No. 2005-292740, an upper limit of ISO speed rating is set, and a photographer's technical level is entered.
However, it is difficult to select a technical level because it differs from one individual to another. Further, an incorrect entry of the setting or a failure to change the setting causes an inevitable increase in noise. One of the reasons a noise component may be superimposed on image data is the existence of external noise entering an image pickup element, such as magnetic noise, radio-frequency noise, or electrostatic noise, or the existence of power supply line noise or signal line noise supplied to an image pickup element or peripheral integrated circuits (ICs).
For example, when a motor located near an image pickup element is activated during photoelectric conversion of the image pickup element, magnetic noise is generated by the motor and current ripple noise is generated by power supply to activate the motor. Such magnetic noise and current ripple noise may influence the photoelectric conversion result. Consequently, a noise component is superimposed on the photographed image.
If an image pickup element of a camera receives a small amount of light reflected from an object, the number of noise components becomes larger than the number of signal components. In other words, the S/N ratio is low. To obtain a bright image, the ISO speed rating is set high.
By setting the ISO speed rating high, an entirely bright image is obtained. However, the noise components are also amplified accordingly, resulting in a noise-dispersed image.
In a situation where an image is captured with a high ISO speed rating or information concerning such an image is handled, it is desirable to reduce noise components.